The present invention relates to a method of making silica, especially superfine silica, through a carbonization reaction with water glass and carbon dioxide as raw materials.
Silicon dioxide (SiO2.nH2O) known as white carbon black (silica) is a widely used raw material in inorganic chemicals. It is used mainly as reinforcing fillers for the sake of its excellent merits of acid resistance, base resistance, high temperature resistance and stability in its chemical properties. Generally, the mean particle size of superfine silica is in the range of from 1 nm to 100 nm. Because of a narrow particle size, huge specific surface area, and strong or weak hydrogen bonds formed between surface silanol group and active silane bond in the molecule, superfine silica has special characteristics such as good electric insulation, hygroscopicity, extinction and dispersivity. Therefore superfine silica is widely used in the field of rubber, plastic, paper-making, ink and daily-used chemicals and the like.
Although there are many methods of preparing silica, carbonization is a commonly used method of preparing silica nowadays. In the carbonization method, water glass is reacted with carbon dioxide (CO2) in carbonization reaction, then the resultant is acidified, aged, filtered, washed, dried, and ground to make the final product, silica. Although the raw materials, i.e., water glass and CO2, are available easily, the process is simple and the operation is easy, the resulted product from this method has such shortcomings that particle size is large, the distribution of particle size is not uniform, and purity is not high, which resist the wider use of the method. In the process of carbonization to make silica, the carbonization step is the most critical section, which can influence the qualities of final product. At the same time, reactor carrying out the carbonization reaction is also important. As reported in an article entitled xe2x80x98Engineering Analysis of White Carbon Black Preparation of Carbonizationxe2x80x99 (INORGANIC CHEMICALS INDUSTRY 1986 No. 2), during the process of carbonization in tank reactor with stirring, the stirring speed is 200 rpm. Higher stirring speed will cause liquid to splash easily, which cannot promote the reaction, but gives to a negative result. The reaction will last 6 hours. After completion of the reaction, the calculated conversion is only 71.5% and pH value is 9 to 10. In order to increase the conversion to 95%, the reaction time of not less than 95.6 hours is required.
A paper entitled xe2x80x9cDiscussion on Silica Preparation of Carbonization Technology in Kiln Gas Bubble Towerxe2x80x9d (INORGANIC CHEMICALS INDUSTRY 1988 No.2) discloses a carbonization reaction carried out in a bubble tower. The bubble tower simplified the apparatus and raised the capability of production in contrast to the conventional stirring reaction tank. Nevertheless, it also takes 4-5 hours to finish the reaction.
On the basis of the prior art works, the inventors of the invention surprisingly find that silica, especially superfine silica, can be obtained through carbonization reaction under a high gravity field. The method according to the invention can reduce reaction time of carbonization, and increase reaction efficiency, and it can improve the qualities of silica product. The mean particle size of silica prepared by the invention is small, only 1 to 100 nm. Moreover, the distribution of the resulted silica according to the invention is uniform and narrow, and its dispersiveness is excellent.
So the purpose of this invention is to provide a method of preparing silica, which can reduce reaction time of carbonization.
The invention provides a method of making silica, especially superfine silica. The method comprises the following steps of: proceeding a carbonization reaction using water glass and CO2 as raw materials under high gravity field, conducting post-treatments for the resultant in a conventional manner for silica product, and collecting resulted silica, especially superfine silica, more especially nano-silica. In general, the post-treatments mentioned above comprise acidifying, insulating aging, filtering, washing and/or drying as well as grinding optionally. Silica product obtained according to the invention is superfine. The mean particle size of the resulted silica is in the range of about 1 to about 100 nm, preferably about 10 to about 50 nm, more preferably 15 to about 30 nm.